Fix for proplem where polygons partly outside the drawing window would get
one vertex clamped to the opposite window side. This bug could also cause
a hardlock. (Bugzilla #2066, Yann Vernier)
#define GET_VIEWPORT_MAT() VIA_CONTEXT(ctx)->ViewportMatrix.m
#define GET_TEXSOURCE(n) n
#define GET_VERTEX_FORMAT() VIA_CONTEXT(ctx)->vertexSize
-#define GET_VERTEX_SIZE() VIA_CONTEXT(ctx)->vertexSize
+#define GET_VERTEX_SIZE() (1<<GET_VERTEX_STRIDE_SHIFT())
#define GET_VERTEX_STORE() VIA_CONTEXT(ctx)->verts
#define GET_VERTEX_STRIDE_SHIFT() VIA_CONTEXT(ctx)->vertexStrideShift
#define GET_UBYTE_COLOR_STORE() &VIA_CONTEXT(ctx)->UbyteColor